Present natural gas engine systems may experience instability in the engine speed which is due to the manner in which the fuel command for the engine is calculated. A fuel command for a natural gas engine may be determined based on several engine parameters including, desired and actual engine speed, inlet manifold pressure, and manifold temperature. Depending on the sequence of events, or calculations, there may be a significant delay between the time the desired and actual engine speeds are sensed, and the time the fuel system responds to the difference between the actual and desired engine speeds. The delay is due in part to the calculation of a throttle command for controlling the position of the throttle, and then measuring the resulting manifold pressure and temperature. A change in throttle position will result in a change in the volume of the air/fuel mixture that is delivered to the manifold, which in turn results in a change in the inlet manifold pressure and temperature. However, the inlet manifold pressure and temperature do not instantaneously reach a steady state value in response to a change in the throttle command. Therefore, the fuel command calculated does not adequately account for the desired and actual engine speeds, resulting in engine speed oscillations of 10-15 r.p.m., at low frequencies, which eventually results in engine instability.
The present invention is directed to overcoming one or more of the problems set forth above.